Valence transition

In conclusion, the valence classification of Yb compounds, based on the SIC-LSD total energies, maps very well onto the physical properties observed experimentally. In particular, this allows to identify the third group of compounds as the heavy-fermion and mixed-valent systems on the trivalent side, and, on the divalent side, those systems that are likely to imdergo pressure-induced valence transitions. 

At ambient conditions, the Yb monopnictides and monochalcogenides crystallize in the B1 structure. As outlined, the Yb pnictides are all foimd to be well described by the nominally trivalent scenario, where the effective valence varies from 2.88 in YbN to 2.69, 2.63, and 2.53 in YbP, YbAs, and YbSb, respectively. Experimentally, the position of the/ band is found 0.2 eV above the Fermi level in YbN, YbP, and YbAs (Degiorgi et al., 1990,1993). Other experiments have revealed heavy-electron behaviour in Yb pnictides (Ott et al., 1985 Sakon et al., 1992 Takeda et al., 1993), but this can be a reflection of sample non-stoichiometry (Degiorgi et al., 1990,1993). The discrepancy between the present electronic structure and the pictme provided by Degiorgi et al. (1990,1993) can be due to the LSD approximation, since the position of the narrow band in the theory is solely determined by the LSD potential (no correlation correction). LDA - - U calculations on YbN (Larson et al., 2007) include a positive correlation shift of the unoccupied f-states that leads to an ideal trivalent Yb ion in accordance with Degiorgi et al. (1990,1993). 

In contrast to the pnictides, the equilibrium volumes of the Yb chalcogenides are accurately described assuming the divalent/ configuration for Yb. As pressme is applied to the Yb chalcogenides, the configuration becomes more and more favourable, and eventually a transition to an intermediate valence state occurs. Valence transitions in lanthanide systems will be discussed in the next section. 

When pressure is applied to lanthanide systems, the interaction of the electrons generally increases and at some point, it becomes advantageous for the f-electrons to contribute more actively to the bonding, that is the effective valence increases. 

In the SIC-LSD formalism, this happens when localization scenarios of different/ configurations become close in energy. In the present T = 0 K theory, only discontinuous pressure-induced transitions can be described, while experiments often, but not always, observe continuous transitions, signalled by anomalous softening of the pV-curve. 

---

Most electronic valence transitions shift to longer wavelengths at higher pressures drat is, the gap between the highest occupied orbital and lowest unoccupied orbital tends to decrease upon compression. The rates of shift usually are larger (1) for pure materials than for solutes in a solvent and (2) for stronger (more allowed) transitions. However, these correlations are not quantitative, and many transitions shift in the opposite... 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

LOMI [Low oxidation metal ions] A process for decontaminating parts of nuclear reactors by washing with aqueous solutions of low-valency transition metal ions. Developed at the Berkley laboratories of the UK Atomic Energy Authority in the early 1980s. 

With regard to the latter point, the absence of a mixed valence transition in the oxidized low coverage polymer case is an important point. No mixed valence transtiion was observed over the whole range of oxidation (0-100%) studied. This indicates that whereas the D22+ aggregate is stable under these conditions, the mixed valent dimer analog, D2+, is not. At least in these polymeric matrices, therefore, the stoichiometric requirement for observation of the mixed valence state appears to involve (D2+)n where n > 2. 

D.Z. Noreus, Properties of formal low-valence transition metal-hydrogen complexes in MgjNiH and Na/dH, Phys. Chem. NE 163 (1989) 575-578. 

The hydroxyl radical is a powerful oxidant, having a reduction potential of 2.7 V in acidic solution. In neutral solution, where the free energy of neutralization of OH by H3O is not available, the reduction potential decreases to 1.9 V (Table 2). Several inorganic anions and low-valency transition metal ions readily undergo one-electron oxidation by reaction with OH, which is often represented as a simple electron transfer ... 

The high stability of TATB favors its use in military and civilian applications where insensitive high explosives are required. In addition to its applications as a HE, TATB is also used to produce the important intermediate benzenehexamine which has been used in the preparation of ferromagnetic organic salts and in the synthesis of new heteropolycyclic molecule such as 1,4,5,8,9,12-hexaazatriphenyl-ene (HAT) that serves as a strong electron acceptor ligand for low-valence transition metals. 

It is generally accepted that valency transitions of cations are connected with the redox mechanism. It is obvious therefore, that activity and selectivity demand that the cation in the active site has the right oxidation state before the hydrocarbon is adsorbed, and that it is effectively reoxidized afterwards. 

An absorption (1030 nun) found in the near-infrared spectrum of this complex arises from a mixed valence transition. Light-induced meial-to-metal charge transfer was predicted by Hush56 for systems of this type before it was observed experimentally. Further, his theory relates the energy of absorption to that required for thenral electron transfer (hv = 4 x Ec) and from this it is possible to calculate the thermal electron transfer rale constant (5 x (08 s-1).57... 

C. G. Chatgilialoglu and M. Guerra,/. Am. Chem. Soc., 112, 2854 (1990). MSXa Study of Absorption Spectra of Free Radicals. Characterization of Rydberg and Valence Transitions in Alkyl Derivatives of Group 14 Centered Radicals. 

Low-valence transition metal complexes of a-diimine ligands are highly colored because of the presence of low-energy metal to a-diimine charge transfer (MLCT) transitions. For a series of d6-M(CO)., (a-diimine) (M=Cr,Mo,W) and d8- M (CO)3 (a-diimine) (M =Fe, Ru) complexes, we have studied the spectroscopic and photochemical properties (1-10). The a-diimine ligands used are 1,4-diaza-1,3-butadiene (R-DAB), pyridine-2-car-baldehyde-imine (PyCa), 2,2 -bipyridine (bipy) or 1,10-phenanthroline (phen) molecules. A close relationship was deduced between the photochemical behavior of these complexes and their resonance Raman (rR) spectra, obtained by excitation into the low-energy MLCT band. 

---